1. Field of The Invention
This invention relates to a method of cancelling echoes to achieve simultaneous two-way data transmission ("full-duplex") over a two-wire circuit.
2. Description of the Prior Art
It is known in the art to utilize echo cancellation for the purposes of simultaneous transmission of data in opposite directions over a two-wire circuit. Such a known technique has been disclosed in an article entitled "Digital Echo Cancellation for Baseband Data Transmission" in IEEE TRANSACTIONS ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, VOL. ASSP-27, No. 6, DEC. 1979, pages 768-781.
An echo canceller is provided with an adaptive digital filter to simulate echo impulse response. The echo canceller produces an echo replica, which corresponds to input data sequence to be transmitted, for use in suppressing or nullifying an echo which has leaked into itself via a hybrid coupler which functions as a four-wire to two-wire interface. Each tap coefficient of the adaptive digital filter is successively adjusted or updated by correlating data to be transmitted and a difference signal which is obtained by the subtraction of an echo replica from a mixed signal including an echo as well as a received signal.
This technique of successively updating the tap coefficients of the adaptive filter (viz., echo canceller convergence algorithm), has been described in the above-mentioned article as "stochastic iteration algorithm" and "sign algorithm".
In order to realize a full-duplex operation of data transmission over the two-wire circuit, it is necessary to make use of LSI (Large Scale Integration) techniques, wherein A/D (Analog-to-Digital) and D/A (Digital-to-Analog) converters are required. How many bits are required by a D/A converter depends on the conditions required by a system in which the converter is used. In the case where the echo canceller is applied to subscriber's transmission systems by way of example, the D/A converter requires approximately 12 bits. On the other hand, the number of bits required by an A/D converter depends on the convergence algorithm in addition to the conditions required by the system used. In the case where the echo canceller is applied to the subscriber's transmission systems using the stochastic iteration algorithm, the A/D converter requires about 8 bits. Contrarily, the sign algorithm needs only 1 bit for A/D conversion. Further, the sign algorithm is well suited for LSI manufacturing.
The sign algorithm, however, encounters a problem in that the polarity of the difference signal is utilized to update the tap coefficients of the adaptive digital filter. More specifically, the adaptive digital filter is no longer able to operate correctly in the event that the polarity of a residual echo is not identical to that of the difference signal. For example, in the case where biphase codes are used as transmission codes, the above-mentioned problem arises when the level of residual echo reaches approximately the level of the received signal. A solution to this difficulty will be described with reference to FIG. 1 which shows, in block diagram form, a known echo canceller using the sign algorithm.
The FIG. 1 arrangement is provided at either end (west and east) of a two-wire circuit 4. Viz., in the case where the echo canceller shown in FIG. 1 is applied to a subscriber line, one is provided at a subscriber's end and the other at a telephone exchange. For ease in description, it will be assumed that transmission is implemented through baseband data.
In FIG. 1, a binary data sequence is applied, via an input terminal 1, to a transmitter 2 and also to an adaptive digital filter 8. The binary data sequence is converted, at the transmitter 2, into transmission codes which are sent over the two-wire circuit 4 to the opposite echo canceller (not shown) by way of a hybrid circuit 3. A part of the transmitted signal, however, leaks into a low-pass filter 5 (viz., echo) as a result of impeadance irregularity in the hybrid circuit 3. The low-pass filter 5 suppresses frequency components outside a desired signal bandwidth. On the other hand, the signal transmitted from the opposite end enters the low-pass filter 5 via the hybrid circuit 3. As a consequence, the low-pass filter 5 outputs a mixed signal, which contains the received signal and echo and which is applied to a subtracter 10.
As shown, there is a closed-circuit which consists of the subtracter 10, an adder 11, a polarity decision circuit 12, a multiplier 13, the adaptive digital filter 8 and a D/A converter 9. This closed-circuit functions to suppress or nullify the echo present in the mixed signal applied from the low-pass filter 5. The echo cancellation is implemented by producing an echo replica in the adaptive digital filter 8 using an error signal applied thereto. The filter 8 is well known in the art and hence a detailed description thereof will be omitted for brevity.
The echo replica (digital) from the adaptive digital filter, is converted, at the D/A converter 9, into the corresponding analog echo replica. The subtracter 10 subtracts the echo replica from the mixed signal (viz., "received signal"+"echo"). The output of the subtracter 10 is therefore the aforesaid difference signal (viz., "received signal"+"residual echo"), wherein the residual echo represents ("echo"-"echo replica"). The difference signal is applied to the receiver 6, the adder 11 and also to an amplitude controller 14. The receiver 6 extracts clock signals, demodulates the received signal, and generates a reproduced signal which is fed to an output terminal 7.
The amplitude controller 14 is supplied with a random signal from a random signal generator 15, and controls the amplitude of the random signal according to the amplitude or electric power of the difference signal applied from the subtracter 10. The adder 11 receives the amplitude-controlled random signal as well as the difference signal, and add these signals. The added signal is then applied to the polarity decision circuit 12 which detects the polarity thereof. The output of the polarity decision circuit 12 is multiplied at the multiplier 13 by 2.alpha. (where .alpha. is a suitable amplification factor), and is applied as an error signal to the adaptive digital filter 8. In order to assure the correct operation of the adaptive digital filter 8, it is vital to correctly detect the polarity of the residual echo at the decision circuit 12.
As mentioned above, the difference signal (viz., the output of the subtracter 10) includes the received signal. Consequently, if the difference signal is directly applied to the polarity decision circuit 12, the polarity of the residual echo is not correctly determined when the residual echo becomes sufficiently small to an extent that the level thereof approximates the level of the received signal. This means that the adaptive implementation is not carried out at the adaptive digital filter 8. In an effort to eliminate this problem, the random signal with an amplitude approximately equal to the level of the received signal is added to the difference signal. To this end, three components are added, viz., the adder 11, the amplitude controller 14 and the random signal generator 15, as shown in FIG. 1. With this arrangement there is a possibility that the received signal is cancelled. Consequently, there is a possibility that the polarity of the residual echo is correctly determined, which ensures the correct operation of the adaptive digital filter 8.
The echo canceller shown in FIG. 1, however, has encountered a problem that sophisticated and complex level control of the random signal is needed and thus results in complex and bulky hardware.
Further, with the aforesaid known echo canceller using the sign algorithm, the tap coefficients of the adaptive filter are adjusted using the polarity of the incoming error signal. Consequently, the echo canceller has encountered another problem that a large amount of time is required for convergence in that the probability of the random signal cancelling the received signal is low.